A stillbirth occurs when a fetus dies after 20 weeks of pregnancy. It can happen before or during delivery. In 2009, there were an estimated 2.64 million stillbirths worldwide. A cause is unknown in 25 to 40 percent of fetal deaths.

In this study, researchers conducted genetic tests on fetuses that died in 91 unexplained cases of stillbirth and found that 18 (19.8 percent) of them had gene mutations associated with so-called long QT syndrome, according to the study in the April 10 issue of the Journal of the American Medical Association.

The syndrome is a heart disorder that increases the risk for an irregular heartbeat and other heart problems. Long QT syndrome can cause sudden death in young people and is believed to be a factor in up to 10 percent of cases of sudden infant death syndrome (SIDS), the researchers noted in a journal news release.

There have also been suspicions that long QT syndrome may contribute to sudden fetal death. These findings support that theory, said Dr. Lia Crotti, of the University of Pavia in Italy, Dr. Michael Ackerman, of the Mayo Clinic in Rochester, Minn., and colleagues.

The findings add to efforts to learn more about the causes of stillbirth and, if confirmed in further studies, may provide an explanation for the many cases of stillbirth and late miscarriage whose cause is currently labeled as unknown, Dr. Alan Guttmacher, of the U.S. National Institutes of Health, and colleagues wrote in an accompanying journal editorial.